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United States Department of Agriculture

Agricultural Research Service

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Research Project: FOOD SAFETY AND FOOD SECURITY ISSUES ASSOCIATED WITH ACID AND ACIDIFIED FOODS

Location: Food Science Research

2011 Annual Report


1a.Objectives (from AD-416)
The overall objective of this research is to develop methods to prevent the growth of pathogenic and spoilage microorganisms in minimally preserved, brined, and fresh-cut foods and optimizing safety, quality, and sensory attributes specifically through:.
1)development of fundamental knowledge of the biochemistry of bacterial adaptation to acidified environments;.
2)determination, through analysis of gene/protein expression profiles, the responses to intrinsic and extrinsic stressors and, in particular, the effect of oxygen imposed on pathogenic bacteria during production, processing, and storage of acid and acidified foods;.
3)development of data and its use for the development of mechanistic models for growth, survival and inactivation of pathogens.


1b.Approach (from AD-416)
The effect of common food acids and acid preservatives will be evaluated for their relative ability to enhance killing of acid-tolerant food pathogens, particularly Escherichia coli O157:H7, in the absence of oxygen and independent of pH. Work will be carried out at biosafety level 2 (BSL-2) due to the organisms under investigation. Selected acid/pathogen strain combinations will be analyzed using genetic and biochemical analyses to determine the mechanisms by which acids are responsible for killing E. coli and other pathogens. This information will be utilized to identify common metabolic targets for the killing effects of acids and acid preservatives, as well metabolic targets unique to particular acids. Since we have found that oxygen increases the killing rates of acid-tolerant pathogens at low pH, similar investigations will be done to determine the genetic and metabolic responses of acid-tolerant pathogens to acids in the presence of molecular oxygen and oxygen radicals. These results will be used to determine the mechanisms by which oxygen species enhance killing of pathogens in acid and acidified foods. Data from genetic and metabolic experiments will be used to develop mechanistic mathematical models and validate the models that are developed in order to test hypotheses developed from genetic and metabolic investigations of acid-killing and acid resistance of food pathogens.


3.Progress Report
The safety of acidified foods is of concern to the Food and Drug Administration and producers of these products. Some disease causing bacteria are very acid resistant and can survive up to several months in some acidified foods, such as cucumber pickle products, if these foods are not properly treated. Bacterial strains of a common acid resistant organism, Escherichia coli (E. coli) O157:H7, were analyzed to determine how they survive under conditions of acidified foods. It was found that survival and acid resistance varied depending on the source of isolation of the strain. Strains isolated directly from animal sources were more acid resistant than strains isolated from foods or human disease. The interaction of oxygen and components of brine for acidified vegetable products was found to be important in the survival of acid resistant disease causing bacteria under conditions similar to acidified food products. It was found that salt concentration and dissolved oxygen can have synergistic effects and significantly affect survival in acid environments for E. coli O157:H7. Methods for determining how internal metabolites are affected by organic acids were developed, using a two dimensional gas chromatograph with a time-of-flight mass spectrometer detector. This is the final report for Project 6645-41420-005-00D, which has been replaced by new Project 6645-41420-006-00D. For additional information, see the new project report.


4.Accomplishments
1. Organic acids inhibit Escherichia coli (E. coli). During the concluding months of this project, it was determined that a gene controlling the uptake of D-lactic acid by E. coli O157:H7 facilitated the conversion of D-lactic acid (isomer of lactic acid) to pyruvate, which can help the cells survive acid stress in the presence of oxygen. Our hypothesis was that pyruvate reacts with and detoxifies reactive oxygen species in the cell. This result is primarily a basic science result that will benefit the scientific community seeking to understand acid resistance of bacterial pathogens. However, the ultimate beneficiary will be the acidified foods industry, because the new knowledge may lead to novel methods to kill bacterial pathogens combining dissolved oxygen species with organic acids.


Last Modified: 4/16/2014
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